In iOS 4.0, whenever a number appears to be a tracking number, it will show up in blue the same way that anything that appears to be a phone number or address does. If you tap the tracking number, Apple's server automatically figures out which shipping carrier it is and takes you to the tracking information on the shipping carrier's website. It's a useful feature, especially if you check shipments a lot obviously.
i think this worked in previous iOS builds. it worked thru emails for me before if i remember. it is very very useful.
not sure this is ios4 specific as i just recieved an email from apple clicked on the tracking number and it took me to the fedex tracking site. would be nice if it took you to the app